Offshore thermal energy conversion plant

Abstract

To provide a suitable sea-floor established ocean thermal energy conversion plant.SOLUTION: The ocean thermal energy conversion plant can include an operations center located onshore, a seafloor - based structure located offshore, the seafloor - based structure containing a plant evaporator and a plant condenser, and control cables extending between the operations center and plant machinery in the seafloor - based structure. A method of providing electricity can include transmitting a signal from an operations center located onshore to an unmanned structure located offshore and, in response to the signal, operating an evaporator, a condenser, and a pump located in the unmanned structure to generate between 0.5 megawatts and 15 megawatts of electricity within the unmanned structure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Application No. 62 / 880,803, filed July 31, 2019, the contents of which are incorporated herein by reference in their entirety.

[0002] (Technical field) The present disclosure relates to ocean thermal energy conversion and power plants, and more particularly to subsea-based ocean thermal energy conversion and power plants. [Background technology]

[0003] (background) Ocean thermal energy conversion ("OTEC") is a mode of producing renewable energy using solar energy stored as heat in tropical regions of the ocean. Tropical oceans and seas around the world offer unique renewable energy resources. In many tropical regions (approximately 20°N to 20°S latitude), sea surface temperatures remain fairly constant. To a depth of approximately 100 feet, the mean sea surface temperature of ocean waters fluctuates seasonally by 75°F to 85°F or more. In the same regions, deep ocean waters (2,500 ft to 4,200 ft or more) remain a fairly constant 40°F. Thus, tropical ocean structures result in a large warm water reservoir at the surface and a large cold water reservoir in the deep ocean, with a temperature difference between the warm and cold reservoirs of 35°F to 45°F. This temperature difference remains fairly constant throughout the day and night, with only minor seasonal variations.

[0004] The OTEC process uses the temperature difference between ocean surface and deep tropical waters to drive a heat engine and produce electrical energy. OTEC power generation was identified in the late 1970s as a potential renewable energy source with low to zero carbon dioxide emissions due to the energy produced. However, OTEC power plants have low thermodynamic efficiency compared to more conventional high-pressure, high-temperature power plants. For example, using a mean ocean surface temperature of 80°F to 85°F and a constant deep-sea water temperature of 40°F, the maximum ideal Carnot efficiency of an OTEC power plant would be 7.5 to 8%. In practical operation, the total power efficiency of an OTEC power system is estimated to be about half the Carnot limit, i.e., approximately 3.5 to 4.0%. Additionally, analyses conducted by leading researchers in the 1970s and 1980s and documented in "Renewable Energy from The Ocean, a Guide to OTEC," William Avery and Chih Wu, Oxford University Press, 1994 (incorporated herein by reference) indicated that one-quarter to one-half (or more) of the total electricity generated by an OTEC plant operating with a ΔT of 40°F would be required to start the water and working fluid pumps and power other auxiliary needs of the plant. Based on this, the low overall net efficiency of OTEC power plants, which convert thermal energy stored in ocean surface waters into net electrical energy, made them a commercially viable energy production option.

[0005] An additional factor that further reduces overall thermodynamic efficiency is the losses associated with providing the necessary control on the turbine for precise frequency regulation, which introduces pressure losses into the turbine cycle that limit the work that can be extracted from the warm seawater.

[0006] This low OTEC net efficiency, compared to the efficiencies typical of heat engines operating at high temperatures and pressures, has led to the widely held assumption by energy planners that OTEC power is too expensive to compete with more conventional methods of power production.

[0007] In fact, parasitic power requirements are particularly significant in OTEC power plants due to the relatively small temperature difference between the hot and cold water. To achieve maximum heat transfer between the warm seawater and the working fluid, and between the cold seawater and the working fluid, a large heat exchange surface area is required, in addition to high fluid velocity. Increasing any one of these factors can significantly increase the parasitic load on the OTEC plant, thereby reducing net efficiency. An efficient heat transfer system that maximizes energy transfer within the limited temperature difference between the seawater and the working fluid would improve the commercial viability of OTEC power plants.

[0008] In addition to relatively low efficiency with its seemingly inherently large parasitic loads, the operating environment of OTEC plants presents design and operational challenges that also reduce the commercial viability of such operation. As previously mentioned, the hot water required for OTEC heat engines is found at the ocean's surface to depths of 100 feet or less. A consistent source of cold water for cooling OTEC engines is found at depths of 2,700 feet to 4,200 feet or more. Such depths are typically not found in close proximity to settlements or even continents. Offshore power plants are required.

[0009] Environmental concerns associated with OTEC plants are also an obstacle to OTEC operation. Conventional OTEC systems draw large amounts of nutrient-rich cold water from the ocean depths and discharge this water at or near the ocean surface. Such discharges can affect the marine environment in the vicinity of the OTEC plant in positive or adverse ways, affecting fish stocks and reef ecosystems that may be downstream from the OTEC discharge. Summary of the Invention [Means for solving the problem]

[0010] (summary) Aspects of the present disclosure are directed to a subsea-based power plant utilizing an OTEC process, for example, an OTEC plant including an operations center located on land, a subsea-based structure located offshore containing a plant evaporator and a plant condenser, and a control system extending between the operations center and the plant machinery within the subsea-based structure. Embodiments can include one or more of the following features.

[0011] In some embodiments, the OTEC plant includes a primary seawater pipe that extends from a subsea foundation structure to a depth of at least 1,500 feet, the primary seawater pipe being restrained on or slightly above the seabed.

[0012] In some embodiments, the OTEC plant includes a power transmission line extending from the undersea foundation structure across the coastline, the transmission line configured to transmit electricity between 10 kilovolts and 35 kilovolts.

[0013] In some embodiments, the OTEC plant includes a water supply line that extends from the subsea foundation structure to land.

[0014] In some embodiments, the control system comprises a control cable extending between the operations center and the subsea establishment structure.

[0015] In some embodiments, the primary control system is located on an undersea establishment structure that is connected to the utility company's Supervisory Control and Data Acquisition (SCADA) system located on land via a control cable between the operations center and the undersea establishment structure.

[0016] In some embodiments, the plant evaporator and plant condenser are located below the waterline of the subsea established structure.

[0017] In some embodiments, the plant evaporator and plant condenser are located slightly (2 to 4 feet) above the water line of the subsea established structure.

[0018] In some embodiments, the seabed establishment structure extends less than 30 feet above the waterline.

[0019] In some embodiments, the subsea establishment structure has a vertical height, measured from the seabed to its highest top, and the highest top of the subsea establishment structure extends above the waterline by less than 20% of the vertical height of the subsea establishment structure.

[0020] In some embodiments, the subsea establishment structure has a vertical height, measured from the sea floor to its highest top, and the highest top of the subsea establishment structure extends above the waterline by less than 40% of the vertical height of the subsea establishment structure.

[0021] In some embodiments, the subsea establishment structure is installed in a location within a water depth of 45 to 250 feet (e.g., less than 200 feet, less than 150 feet, more than 80 feet, or more than 100 feet), measured at the centerline of the structure.

[0022] In some embodiments, the OTEC plant includes a subsea establishment structure located where the distance between the coastline and the continental shelf edge is between 150 yards and 6,600 yards. In some cases, the subsea establishment structure is located where the ocean floor offshore of the continental shelf edge slopes downward to a depth of at least 1,500 feet within 300 yards of the coastline.

[0023] In some aspects, a method of providing electricity includes transmitting a signal from an operations center located on land to an unmanned structure located offshore, and in response to the signal, operating an evaporator, a condenser, and a pump located within the unmanned structure to generate between 0.5 megawatts and 15 megawatts of net electricity within the unmanned structure. Embodiments can include one or more of the following features.

[0024] In some aspects, a method of providing electricity includes transmitting a signal from an operations center located on land to an inhabited structure located offshore, and, in response to the signal, operating an evaporator, a condenser, and a pump located within the inhabited structure to generate between 0.5 megawatts and 15 megawatts of net electricity within the inhabited structure. Embodiments can include one or more of the following features.

[0025] In some embodiments, the method includes pumping seawater from a depth of at least 1,500 feet to the uninhabited structure.

[0026] In some embodiments, the method includes transmitting electricity from the unmanned structure to land.

[0027] In some embodiments, the method includes pumping water from the uninhabited structure to land.

[0028] In some embodiments, transmitting the signal includes transmitting the signal from an operations center on land to the offshore unmanned structure through a control cable extending between the operations center and the subsea established structure.

[0029] In some aspects, a method of providing electricity includes transmitting signals between a utility operations control center located on land and a manned operations control center located on a subsea-founded structure located offshore, and operating an evaporator, a condenser, and a pump located within the manned structure in response to the signals from the utility operations control center to generate between 0.5 megawatts and 15 megawatts of electricity within the manned structure. Embodiments can include one or more of the following features.

[0030] In some embodiments, the method includes pumping seawater from a depth of at least 1,500 feet to the manned structure.

[0031] In some embodiments, the method includes transmitting electricity from the manned structure to land.

[0032] In some embodiments, the method includes pumping water from the manned structure to land.

[0033] In some embodiments, transmitting the signal includes transmitting the signal from an operations center on land to a manned structure offshore through a control cable extending between the operations center and the subsea established structure.

[0034] In some embodiments, transmitting the signal includes transmitting the signal from an operations center on land to a manned structure offshore through a control cable extending between the operations center and the subsea established structure.

[0035] Subsea-based OTEC plants can be implemented in combination with an onshore operations center and interconnections to the onshore switchyard / electrical grid, along with an unmanned offshore plant housing equipment such as evaporators, condensers, pumps, and generators. The operations center is often co-located with the interconnections to the switchyard / electrical grid. The unmanned offshore plant is designed to reduce maintenance requirements by making as much of the existing offshore plant equipment maintenance-free as possible. This will likely result in more robust monitoring, command, and control systems and simpler, but more reliable, equipment, resulting in higher capital costs but lower maintenance and labor costs.

[0036] For example, marine coating systems can be applied throughout. Vibration sensors can be placed on all of the rotating machinery, allowing for condition-based rather than scheduled maintenance. An automatic reverse-flow seawater strainer between the seawater pump and the heat exchanger enclosure captures and removes debris that could clog, foul, and reduce the heat exchanger's performance. Seawater and ammonia piping crossovers with isolation valves allow the power plant to operate continuously at near-full output capacity even if one pump, heat exchanger enclosure, or ammonia turbine-generator needs to be shut down for maintenance. To reduce corrosion, the exterior structures of the flat-sided structure—i.e., boat docking platform and ramp, lifeboat suspension mast, handrails and stairs to the open deck, and lighting fixtures—are made from non-corrosive materials. The bodies of the seawater pump and strainer can be made from austenitic stainless steel. The work area may be totally enclosed and air-conditioned, so that only low-maintenance, watertightly enclosed lights, as required by International Maritime Organization conventions, are located on the exterior of the structure. Doors and hatches exposed to sun and waves may be limited to two cargo doors on each side of the main deck and the door to the boat docking. All cargo doors open outward so that if the ocean rises due to a storm surge and waves hit the closed doors, the seals compress the interior of the structure and prevent water from entering.

[0037] Additionally, high reliability items (e.g., seawater strainers, seawater pumps, ammonia pumps, HVAC fans and cooling coils, start-up and emergency diesel generators, LED and fiber optic lighting, variable frequency drives and motors, fire pumps, watertight doors and hatches, instruments and gauges, alarm and control systems) can be established in the offshore structure, while lower reliability and higher maintenance items (e.g., step-up transformers and storage batteries) can be located in the onshore interconnection facility.

[0038] Systems in a manned offshore plant would typically be controlled on-board the structure during normal conditions, but under abnormal conditions may be controlled from an onshore operations center, allowing the plant to continue operating when other onshore power generation systems need to be shut down, thereby providing power to shore during an emergency. Systems in an unmanned offshore plant would be controlled from an onshore operations center under normal and emergency conditions. An unmanned configuration can reduce operating costs because fewer workers need to travel across the ocean to the plant. A manned configuration can reduce operating costs because workers can be housed for extended periods of time, performing routine operations and maintenance between shift changes.

[0039] Subsea-based OTEC plants can be implemented with most or all plant machinery located below the waterline. This configuration can reduce the structure-borne and airborne noise emissions associated with some OTEC plants. Locating pumps below sea level within an OTEC plant reduces parasitic pumped power, thereby making more power available for transmission to shore.

[0040] The low headroom requirement allows a subsea-based OTEC plant to be constructed with the majority of the structure also located below the waterline, reducing the plant's visual impact. This feature is particularly important in locations such as remote resorts sited to take advantage of natural beauty. The low profile above ocean surface in turn reduces the height of safety lights and communication antennas, thus reducing the potential impact on aircraft operations while providing coastal aids to navigation for fishermen and pleasure boat crews.

[0041] Some plants are built on land, with portions of their structure below the waterline and sealed against storm surges and waves. These plants can be subsea-based plants that are moved to artificial inlets, their seabed flush with the adjacent seabed, and their entrances can be closed with protective breakwaters. These plants can be sited so that after they reach the end of their useful life, they can be refloated and removed and replaced by upgraded versions.

[0042] In a subsea-based OTEC plant, stresses on seawater pipe connections are reduced relative to floating OTEC plants: connections on a subsea-based OTEC plant can be fixed and simply flanged, rather than configured to compensate for the motion and resultant forces of both the floating plant and the pipes suspended from the floating plant in the water column.

[0043] As used herein, the term "seabed establishment" includes structures that are fixed to the sea floor.

[0044] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. The present specification also provides, for example, the following: (Item 1) 1. An ocean thermal energy conversion plant comprising: An operations center located on land, an offshore subsea foundation structure, the subsea foundation structure containing a plant evaporator and a plant condenser; a control system extending between the operations center and plant machinery within the subsea establishment structure; An ocean thermal energy conversion plant comprising: (Item 2) 2. The ocean thermal energy conversion plant described in item 1, comprising a primary seawater pipe extending from the seabed establishment structure to a depth of at least 1,500 feet, the primary seawater pipe being disposed on the seabed. (Item 3) Item 1. The marine thermal energy conversion plant according to item 1, further comprising a transmission line extending from the subsea established structure across the coastline, the transmission line being configured to transmit electricity at 10 kilovolts to 35 kilovolts. (Item 4) Item 1. A marine thermal energy conversion plant according to item 1, comprising a water supply line extending from the subsea establishment structure to land. (Item 5) 2. The marine thermal energy conversion plant described in item 1, wherein the control system comprises a control cable extending between the operation center and the subsea installation structure. (Item 6) 2. The marine thermal energy conversion plant according to claim 1, wherein the plant evaporator and the plant condenser are located below the waterline of the subsea-based structure. (Item 7) 2. The ocean thermal energy conversion plant of claim 1, wherein the subsea foundation structure extends less than 30 feet above the water line. (Item 8) 2. The marine thermal energy conversion plant of claim 1, wherein the subsea establishment structure has a vertical height measured from the seabed to its highest top, and the highest top of the subsea establishment structure extends above the waterline by less than 20% of the vertical height of the subsea establishment structure. (Item 9) Item 1. The marine thermal energy conversion plant according to item 1, wherein the subsea installation structure is installed at a location within a water depth of 50 to 250 feet (e.g., less than 200 feet, less than 150 feet, more than 80 feet, or more than 100 feet). (Item 10) 2. The ocean thermal energy conversion plant according to item 1, wherein the seabed installation structure is installed in a location where the distance between the coastline and the outer edge of the continental shelf is 150 yards to 6,600 yards. (Item 11) Item 11. The ocean thermal energy conversion plant described in item 10, wherein the seabed establishment structure is installed in a location where the ocean floor offshore of the outer continental shelf slopes downward to a depth of at least 1,500 feet within 8,000 yards of the coastline. (Item 12) 1. A method of providing electricity, the method comprising: transmitting a signal from an operations center located on land to an unmanned structure located offshore; In response to the signal, operating an evaporator, a condenser, and a pump located within the unmanned or manned structure to generate between 0.5 megawatts and 15 megawatts of electricity within the unmanned structure. A method comprising: (Item 13) Item 13. The method of item 12, comprising pumping seawater from a depth of at least 1,500 feet to the uninhabited structure. (Item 14) Item 13. The method of claim 12, further comprising transmitting electricity from the unmanned structure to land. (Item 15) Item 13. The method of claim 12, comprising pumping water from the uninhabited structure to land. (Item 16) 13. The method of claim 12, wherein transmitting a signal includes transmitting a signal from the operations center to the unmanned structure through a control cable extending between the operations center and a subsea established structure. (Item 17) 1. An ocean thermal energy conversion plant comprising: an offshore subsea foundation structure, the subsea foundation structure containing an evaporative heat exchanger, a condensing heat exchanger, and a control center; a transmission line extending from said subsea foundation structure across the coastline to an onshore interconnection facility; An ocean thermal energy conversion plant comprising: (Item 18) Item 18. The ocean thermal energy conversion plant according to item 17, comprising a primary seawater pipe extending from the seabed foundation structure to a depth of at least 1,500 feet, the primary seawater pipe being disposed on the seabed. (Item 19) Item 18. The marine thermal energy conversion plant according to item 17, wherein the subsea installation structure has a substantially octagonal shape when viewed from above. (Item 20) Item 18. The marine thermal energy conversion plant according to item 17, wherein the subsea foundation structure has a first deck located above mean high tide level and a second deck located below mean high tide level. (Item 21) 21. The marine thermal energy conversion plant according to item 20, wherein the condensing heat exchanger and the evaporating heat exchanger are located on the first deck. (Item 22) 21. The marine thermal energy conversion plant according to item 20, comprising a pump configured to pump cold seawater and warm seawater through a supply pipe and a return pipe, the pump being located on the second deck. (Item 23) Item 18. The ocean thermal energy conversion plant of item 17, wherein the transmission line is configured to transmit electricity at approximately 10 kilovolts to 35 kilovolts to the onshore interconnection facility. (Item 24) Item 18. The ocean thermal energy conversion plant of item 17, wherein the seabed foundation structure extends less than 30 feet above the mean high tide level. (Item 25) 18. The ocean thermal energy conversion plant of claim 17, wherein the subsea establishment structure has a vertical height, measured from the sea floor to its highest top, and wherein the highest top of the subsea establishment structure extends above the mean high tide level by less than 40% of the vertical height of the subsea establishment structure. (Item 26) Item 18. The marine thermal energy conversion plant according to item 17, wherein the subsea installation structure includes accommodation areas for crew members. (Item 27) Item 18. The ocean thermal energy conversion plant according to item 17, wherein the subsea installation structure is approximately three times as wide as it is high. (Item 28) Item 18. The ocean thermal energy conversion plant according to item 17, wherein the condensing heat exchanger and the evaporating heat exchanger are modular. (Item 29) Item 18. The marine thermal energy conversion plant according to item 17, wherein the subsea installation structure is installed at a location within a water depth of 30 to 180 feet. (Item 30) 1. A method of providing electricity, the method comprising: transmitting control signals from a control room of the submarine-established structure; in response to the signal, operating an evaporative heat exchanger, a condensing heat exchanger, and a pump located within the submarine-established structure to generate between 0.5 megawatts and 15 megawatts of electricity within the submarine-established structure; transmitting electricity via transmission lines to onshore interconnection facilities; A method comprising: (Item 31) 31. The method of claim 30, comprising pumping seawater from a depth of at least 1,500 feet to the subsea establishment structure. (Item 32) Item 31. The method of item 30, wherein electricity of approximately 10 kilovolts to 35 kilovolts is transmitted to the onshore interconnection facility. [Brief explanation of the drawings]

[0045] [Figure 1] FIG. 1 is a plan schematic diagram of an exemplary subsea based OTEC plant. [Figure 2] FIG. 2 is a side schematic view of the offshore portion of the subsea based OTEC plant of FIG. [Figure 3] FIG. 3 is a plan view of the evaporator deck of the subsea-based OTEC plant of FIG. [Figure 4] FIG. 4 is a plan view of the condenser deck of the subsea OTEC plant of FIG. [Figure 5]FIG. 5 is a schematic diagram of an aerial view of a second exemplary subsea based OTEC plant. [Figure 6] FIG. 6 is a side schematic view of the subsea OTEC plant of FIG. [Figure 7] FIG. 7 is a schematic side view of the subsea foundation structure portion of the OTEC plant of FIG. [Figure 8] FIG. 8 is a plan view of the first deck of the subsea-based OTEC plant of FIG. [Figure 9] Figure 9A is a schematic diagram of the heat exchanger of the subsea OTEC plant of Figure 5 with the rack of the heat exchanger array removed. Figure 9B is a schematic diagram of the rack of the heat exchanger array of the heat exchanger of Figure 9A. [Figure 10] FIG. 10 is a plan view of the control space and accommodation space on the first deck of FIG. [Figure 11] FIG. 11 is a plan view of the second deck of the subsea-based OTEC plant of FIG. [Figure 12] FIG. 12 is a schematic diagram of the shore-side view of the subsea-based OTEC plant of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0046] (Detailed explanation) Subsea-based OTEC plants can provide highly survivable platforms for coastal environments. Such plants are particularly well suited to locations with shallow, narrow shelves and seawalls that drop rapidly to cold-water depths. Such locations include, for example, numerous sites in the Caribbean, Pacific, and Indian Oceans. The high survivability of subsea-based structures also makes them particularly well suited to locations subject to severe storms.

[0047] There are many island regions in the tropics that could benefit from base load electricity generated by OTEC power plants. Many of these islands have small populations of permanent residents and / or visitors, with low total power demands of 1.5 MW to 5.0 MW. For example, some of the "archipelagos" in the Bahamas have a permanent population of approximately 2,000 to 6,000 people, with peak power demands of 1.5 MW to 10.0 MW. This demand could be supplied by an OTEC plant, but is too small to justify the capital cost of a large offshore platform with its supporting infrastructure for a spar-based OTEC plant.

[0048] Subsea-based OTEC plants can reduce costs and reef damage associated with shore-based OTEC plants. Subsea-based OTEC plants do not require the installation of warm and cold seawater intake and return pipes across the reef and shoreline associated with shore-based OTEC plants. Such pipes increase capital costs and, in some cases, reef damage to shore-based OTEC plants. Subsea-based OTEC plants also do not require multiple anchor points and chains sweeping across the reef associated with floating OTEC plants moored close to shore. Subsea-based OTEC plants can also be installed in locations lacking a shelf wide enough to accommodate an eight-point mooring spread for an OTEC barge.

[0049] 1, an exemplary subsea-based OTEC plant 100 includes an operations center 110 located on land, a subsea-based foundation structure 112 located offshore, and a control system 113 extending between the operations center 110 and the subsea-based foundation structure 112. The subsea-based foundation structure 112 contains the plant evaporator and condenser, pumps (e.g., hot and cold water pumps), and turbine generators driven by working fluids heated and cooled by the evaporator and condenser, respectively.

[0050] The control system 113 of the OTEC plant 100 includes a control station in the operations center 110, control cables 114 extending between the operations center 110 and the subsea foundation structure 112, and remote actuators in the subsea foundation structure 112 operable to control machinery in the subsea foundation structure 112. Some OTEC plants can be implemented using other approaches for remotely operating machinery in the subsea foundation structure 112. For example, some OTEC plants use radio frequency transmission of control signals from the operations center 110 to the subsea foundation structure 112 instead of, or in addition to, transmitting control signals through the control cables 114.

[0051] The exemplary OTEC plant 100 also includes a transmission line 116 that extends from the subsea foundation structure across the shoreline 118. The transmission line 116 is configured to transmit electricity between 10 kilovolts and 33 kilovolts. In some OTEC plants, an operations center is often co-located with a switchyard / interconnection to the electrical grid 120, and the transmission line 116 extends between the subsea foundation structure 112 and the operations center 110. For example, operations such as power conditioning can be performed at an onshore switchyard. In some OTEC plants, the transmission line 116 does not extend to the operations center 110, but rather to a separate switchyard / interconnection to the electrical grid.

[0052] Both the transmission lines 116 and the control cables 114 are laid on the sea floor and lightly covered with rubble or special protective pads in the illustrated OTEC plant 100. This approach is expected to protect the control cables 114 and transmission lines 116 while also limiting damage to the sea floor and reefs.

[0053] In the illustrated OTEC plant 100, a single operations center 110 controls a single subsea foundation structure 112. In some systems, a single operations center 110 is connected to and controls multiple subsea foundation structures 112. Similarly, some systems are implemented with one or more backup operations centers 110 to provide redundancy.

[0054] A primary seawater pipe 122 extends from the seafloor foundation structure 112 across the continental shelf edge 124 to a depth of at least 1,500 feet. The primary seawater pipe 122 is disposed on the sea floor. In the exemplary OTEC plant 100, the primary seawater pipe 122 used for water intake is separate from the primary seawater pipe 122 used for water discharge. In some OTEC plants, the primary seawater pipes 122 for water intake and discharge are separate pipes located in the same location. In some OTEC plants, cold water intake and discharge are provided by a single pipe with at least two separate flow channels.

[0055] Subsea-floor OTEC plants are well suited for locations with shallow, narrow shelves and seawalls that drop rapidly to cold-water depths. Locations where the distance D1 between the coastline 118 and the shelf edge 124 is between 150 yards and 6,600 yards are suitable for the installation of the subsea-floor establishment structure 112. The subsea-floor establishment structure 112 is installed close to the shelf edge 124 at a point where the ocean floor offshore of the shelf edge slopes downward to a depth of at least 1,500 feet within a maximum distance of 15 miles from the coastline. The subsea-floor establishment structure 112 is set back at least 80 yards from the shelf edge to avoid fracturing the seabed strata near the shelf edge. For example, the distance D2 between the coastline and the 1,500-foot seabed contour 126 is 600 yards, and the distance D3 between the seabed establishment structure 112 and the continental shelf edge is 200 yards at one point considered for the seabed establishment OTEC plant.

[0056] The subsea establishment structure 112 can be constructed as a steel frame structure set on a steel or concrete crib that is set and anchored to the seabed. The structure would consist of a crib and two steel decks that would rise approximately 60 feet above the seabed and be partially or fully submerged. A double-walled steel maintenance trunk rising above the ocean surface would allow for periodic inspection and maintenance, as well as equipment removal when necessary. The crib and foundation could be formed from high-strength pre-cast concrete constructed on land, floated to the location, and set on the seabed. Alternatively, the crib could be prefabricated from steel and welded or bolted to the seabed of a structure that would be filled with concrete pumped from the surface after the crib is positioned and set on the seabed.

[0057] Referring to Figures 2-4, an exemplary subsea establishment structure 112 includes an evaporator deck 128 and a condenser deck 130 formed around an access trunk 132 with a large central well. A buttress bracket 134 stiffens the access trunk 132 against moment forces from wave strike. A concentric pipe structure 136 can provide double-wall protection for the access trunk 132. The highest deck of the subsea establishment structure 112 is an upper deck 138 with a large watertight hatch sealing the access trunk 132. The double-walled steel maintenance access trunk 132 rises above sea level by a height h1 under calm conditions at mean high tide. The height h1 can be between 12 and 30 feet. The height h1, where the highest peak of the subsea establishment structure 112 extends above the waterline, is approximately less than 20% of the overall vertical height h2 measured from the sea floor to the highest peak of the subsea establishment structure.

[0058] A navigation signal 144 (e.g., a light and / or an audio signal) can be mounted on top of the access trunk 132. Because only the trunk access rises above sea level, the subsea establishment structure has a low visual impact. The subsea establishment structure can also be sited to serve as a navigation aid for mariners and aircraft.

[0059] A machinery space housing the plant evaporator and condenser is located below the waterline of the subsea establishment structure 112. Hot water intake and discharge ports 140 are formed in the evaporator deck bulkhead. In the subsea establishment structure 112, the hot water intake and discharge ports 140 are open to the surrounding seawater. In some subsea establishment structures 112, hot water intake and / or discharge piping may be required to control the depth of the hot water intake or discharge. For example, discharge piping can be used to return the hot discharge at an appropriate depth to avoid thermal contamination of the hot water intake. A cold water port 142 provides an attachment point for the primary seawater pipe 122.

[0060] The subsea establishment structure 112 is securely attached to the sea floor at a depth D1 between 50 and 250 feet (e.g., less than 200 feet, less than 150 feet, more than 80 feet, or more than 100 feet). At these depths, divers can inspect, service, and maintain external connections, such as ports, flanged pipe connections, and pipe mooring devices. The subsea establishment structure can be configured with the top of the main structure (e.g., the top of the evaporator deck) at a depth D3 between 50 and 250 feet. This keeps the top of the main structure continuously submerged, placing it directly below the aeration caused by everyday wave action and potentially reducing oxidation that can cause corrosion. This configuration also places the hot water return and cold water intake and return pipe connections well below the impact zone of heavy waves.

[0061] The primary seawater pipe 122 can be formed as described in PCT Application No. PCT / US2013 / 065098, filed October 15, 2013. However, the subsea foundation structure 112 is fixed in place and the primary seawater pipe 122 is placed on the seabed and, optionally, covered with riprap. Because the primary seawater pipe 122 experiences little or no stress at its connection to the subsea foundation structure 112, lower cost HDPE can be used for the pipe material, with a service life of up to 100 years; such pipe with an outer diameter of up to 80 inches is commercially available from Australia, Germany, the United States, and Dubai.

[0062] The exemplary OTEC plant 100 incorporates a four-stage hybrid heat exchange cycle, as described in PCT Application No. PCT / US2013 / 068894, filed November 7, 2013. Other heat exchange cycles and plant configurations can also be used in subsea-based OTEC plants.

[0063] The main portion of the subsea establishment structure 112 is a 70-foot square steel structure with rounded corners made from rounded pipe to provide strength and stiffening. Some structures are octagonal, or rather, square with rounded corners. Along a single side of the structure, there is enough space to accommodate sufficient heat exchanger surface area for all four stages, leaving the remaining space on deck for machinery. For example, the hot water pumps and turbine generators can be on the upper deck, and the condenser, cold water pumps and ammonia recovery tank, and recirculation pumps can be on the lower deck.

[0064] Referring to FIG. 3, an access trunk 132 extends through the center of the evaporator deck 128. Machinery disposed on the evaporator deck includes dual 1.5 megawatt turbogenerator sets 146, pumps 148, and pump variable frequency drives 150. Hot water flows from a hot water intake pipe 152 and a screened opening in the side of a hot water intake plenum 154, across an evaporator heat exchanger 156, and to a hot water return plenum 158. A warm seawater intake 170 includes a mesh screen (to prevent fish entrapment) and has an average inlet velocity of 0.5 feet per second. The mesh screen may have a pore size of approximately 0.5 inches. The warm seawater intake 170 is located at least 10 feet below the mean high tide level 180 (shown in FIG. 2). Hot water flows from the screened intake plenum to the heat exchanger chamber and out through the hot water return plenum. The heat exchangers can be implemented using, for example, the heat exchange plates, cabinets, and systems described in PCT Application Nos. PCT / US2013 / 065004, filed October 15, 2013, PCT / US2012 / 050941, filed August 15, 2012, and PCT / US2012 / 050933, filed August 15, 2012. In contrast to these systems, the heat exchangers in the exemplary subsea-based OTEC plant 100 are oriented for horizontal flow rather than vertical flow. The evaporator deck 128 also includes a fugitive trunk 160 with a vertical ladder and a fugitive trunk 162 with an inclined ladder.

[0065] 4, the condenser deck includes substantially identical features in a complementary layout to the evaporator deck. Dual turbo generators 146 are mounted on the opposite side of the deck from the dual turbo generators 146 on the previous deck. Chilled water flows from a chilled water intake pipe 164 and chilled water intake plenum 166, across a condenser heat exchanger 168, to a chilled water return plenum 170. An ammonia recovery tank 172 and an ammonia recirculation pump 174 are also located on the condenser deck.

[0066] FIG. 5 shows another embodiment of an OTEC plant 500. The subsea foundation structure 512 of the OTEC plant 500 is generally octagonal in shape and fabricated from steel. The octagonal shape helps protect the subsea foundation structure 512 from damage from crashing waves during storm conditions. Additionally, waves can crash over the top of the subsea foundation structure 512 during storm conditions without damaging the structure. The subsea foundation structure 512 is configured to withstand storm conditions for up to 100 years. The OTEC plant 500 includes primary seawater pipes 522 that extend from the subsea foundation structure 512 across the continental shelf edge (shown in FIG. 6 ) to a depth of at least 1,500 feet. The primary seawater pipes 522 are disposed on the seabed. In some embodiments, one or more of the primary seawater pipes 522 may be used for cold seawater intake, while one or more of the other primary seawater pipes 522 are used for discharge. In some embodiments, cold seawater intake and discharge is provided by a single primary seawater pipe 522 having at least two separate flow channels.

[0067] The OTEC plant 500 also includes a transmission line 516 extending from the subsea foundation structure 512 to land. The transmission line 516 carries the power generated in the subsea foundation structure 512 to the interconnection facility 510, where the power can be delivered to the power grid for distribution. The transmission line 516 is buried in the seabed 502 to travel beneath the reef structure on the seabed 502, thereby avoiding possible reef destruction. The transmission line can also be installed to avoid the reef in addition to or instead of being buried. The transmission line 516 may connect to the interconnection facility 510 from underground. For example, the transmission line 516 in FIG. 5 is buried beneath a portion of the seabed 502, a beach, and a road before reaching the interconnection facility 510. Power is delivered from the subsea foundation structure 512 to the interconnection facility 510 through the transmission line 516 at 13.8 kV to 35.0 kV. The power delivered from the subsea foundation structure 512 may be boosted onshore from 33 kV to 69 kV or higher and delivered to the power grid. The average annual net power output of the OTEC plant 500 is approximately 5-15 MW.

[0068] FIG. 6 shows a side schematic view of an OTEC plant 500. A subsea foundation structure 512 is positioned on the seabed 502 near the continental shelf edge 504 and extends above mean high tide level 506. The subsea foundation structure 512 is positioned approximately 30 to 80 feet below mean high tide level. The subsea foundation structure 512 is secured to the seabed 502 by a plurality of anchor piles 508. The anchor piles 508 connect the base 524 (shown in FIG. 7) of the subsea foundation structure 512 down to the dolomite layer below the seabed 502. The anchor piles 508 may have a diameter of 16 to 48 inches.

[0069] The primary seawater pipe 522 extends from the subsea establishment structure 512 across the continental shelf edge 504, following the wall and along the seabed 528 to a depth of at least 1,500 feet. The primary seawater pipe is made of high-density polyethylene (HDPE) and has an inner diameter of approximately 8 feet and an outer diameter of approximately 8.2 feet. Using HDPE pipe is advantageous because HDPE resists fouling by marine organisms, is non-conductive, and does not degrade / decompose in seawater. The primary seawater pipe 522 is anchored to the seabed 502 and 528 using concrete saddle anchors 530 and pendant anchors 531. The concrete saddle anchors 530 and pendant anchors 531 hold the cold and hot water pipes in place during storm conditions. The cold water intake pipe 522 is configured to deliver cold seawater at a temperature of approximately 40°F to the subsea establishment structure 512. Cold water return pipe 523 discharges the spent cold water at a depth near or below the mixed layer, approximately 100-160 yards deep. Warm water return pipe 521 discharges the spent warm water adjacent to it at the same depth as cold water return pipe 523 so that the two streams mix and rapidly assimilate with the surrounding ocean conditions.

[0070] Unlike the unmanned subsea establishment structure 112 of the OTEC plant 100 of FIGS. 1-4 , the subsea establishment structure 512 is operated by a crew within the subsea establishment structure 512. As shown in FIG. 7 , the subsea establishment structure 512 includes a first deck 532, a second deck 534, and a base 536. The base 536 is moored to the sea floor by a plurality of anchor holes 508. The first deck 532 and the second deck 534 house power generation equipment, a control room 552 (shown in FIG. 10 ), and accommodations for the crew of the OTEC plant 500. The first deck 532 extends above mean high tide level, while the second deck 534 is below sea level. The first deck 532 connects to a platform 526 on the exterior of the subsea establishment structure 512. A plurality of small boards may be secured to the platform 526. The small board provides access to shore for the crew living and working within the undersea established structure 512 .

[0071] The first deck 532 extends above the mean high tide level 506 by a height h3, which may be approximately 18 to 30 feet. The subsea establishment structure 512 has a width w1, which is approximately 180 to 240 feet. Each side of the octagonal shaped subsea establishment structure 512, shown as w2 in FIG. 8, is approximately 80 to 95 feet long. The top 520 of the subsea establishment structure 512 is cambered to allow for drainage during storm conditions and to allow waves to crash across the subsea establishment structure 512 more easily.

[0072] FIG. 8 shows a schematic diagram of the first deck 532 of the subsea establishment structure 512. The first deck 532 is divided into three sections: an upper ammonia section 538, an upper main section 540, and a crew section 542. The first deck 532 is approximately two feet above the mean high tide level 506. The upper ammonia section 538 includes a turbine generator 544 configured to generate electrical power. The upper ammonia section 538 is located on the ocean-facing side of the subsea establishment structure 512 so that the ammonia is located as far from shore as possible. Additionally, noise emissions from the turbine generator 544 to shore are reduced. The upper ammonia section 538 is separated from the upper main section 540 by an airlock-style access point.

[0073] Crew area 542 is located on the shoreward side of subsea establishment structure 512. Crew area 542 is set on a raised deck such that a watertight dam exists between the crew area 542 and the machinery spaces of first and second decks 532, 534. The watertight dam serves to raise crew area 542 above upper ammonia area 538 and main area 540. Thus, any water that may be present on the deck of main area 540 is below the crew area level. Main area 540 is equipped with an ammonia sensor, is ventilated, and is maintained at a lower pressure than the upper crew area 542 so that even in the event of a leak, no ammonia gas will enter crew area 542.

[0074] The upper main section 540 includes condensing heat exchangers 546, 547 and evaporative heat exchangers 548, 549, in which ammonia is cooled and heated, respectively. As shown in FIGS. 9A and 9B for heat exchanger 547, each heat exchanger 546-549 includes an outer heat exchanger enclosure 551 that provides physical protection from the upper main section 540. The outer heat exchanger enclosure 551 also provides a flow path for cold seawater and / or warm seawater to flow through. Each heat exchanger 546-549 also includes 4 to 20 racks 553. Each rack is configured to hold multiple arrays 555. Each array is approximately 10 feet long, 29 inches high, and 28 inches wide. The arrays can be used interchangeably in both the condensing heat exchangers 546, 547 and the evaporative heat exchangers 548, 549. Each array holds multiple cartridges. Ammonia flows through the cartridges during operation of the OTEC plant 500. When the heat exchangers are not in operation, the outer heat exchanger enclosure 551 can be opened and one or more racks 553 can be removed for maintenance. The racks 553 can be pulled out from the heat exchangers 546-549 on tracks 550a-b (shown in FIG. 8).

[0075] FIG. 10 shows the crew area 542, which includes space for controlling the OTEC plant 500 machinery and space for the crew to live and rest. The crew that operates the OTEC plant 500 includes approximately 17 members, with a minimum of six members present on the subsea establishment structure 512 at any given time. A control room 552 overlooks the upper main area 540 and contains equipment for monitoring and controlling the flow through the heat exchangers 546-549 and other machinery on the subsea establishment structure 512, as well as the regulation and transmission of power to the onshore interconnection facility 510. Equipment on the second deck 534 may also be controlled from the control room. The crew area 542 also provides access to the exterior of the subsea establishment structure 512 on the platform 526. The platform 526 allows small boats 554a-b to be docked at the subsea establishment structure 512. The boats 554a-b provide the crew with access to shore for normal operations or during emergency evacuation protocols.

[0076] 11 shows second deck 534, which includes three sections: lower ammonia section 556, lower main section 558, and water supply / return section 560. Lower ammonia section 556 includes an ammonia storage tank 562 and an ammonia collection tank 564. Approximately 8,000 gallons of ammonia are stored in ammonia storage tank 562 during operation, and approximately 40,000 gallons of ammonia are used during operation.

[0077] The second deck 534 contains seawater intakes for both cold and warm seawater. The cold water intake ("CSW supply") is located within the water supply / return area 560, while the warm seawater intakes 580, 581 are located on the sides of the subsea establishment structure 512. The warm seawater intakes 580, 581 include plenums containing mesh screens (to prevent fish entrapment) and have an average inlet velocity of 0.5 feet per second or less. The mesh screen may have a pore size of approximately 0.5 inches. The warm seawater intakes 580, 581 are located at least 10 feet below the mean high tide level 506 (shown in FIG. 7). Lower main section 558 includes cold seawater strainers 566, 567 and warm seawater strainers 568, 569, which filter the cold and warm seawater and remove debris, respectively, prior to pumping the seawater through heat exchangers 546-549. Cold seawater pumps 570 and 571 pump the filtered cold seawater into heat exchangers 546 and 547, respectively. Warm seawater pumps 572 and 573 pump the filtered warm seawater into heat exchangers 548 and 549, respectively.

[0078] A start-up generator 574 is located on the shore-facing side of the subsea establishment structure 512. The start-up generator 574 may be, for example, a 2.0 MW diesel generator and is used to begin the power generation process. After the subsea establishment structure 512 has generated enough power to power itself during the power generation process, the start-up generator 574 may be turned off. Storing the seawater pumps 570-573 and the start-up generator 574 on the second deck 534, below the mean high tide level 506, limits airborne noise emissions from the subsea establishment structure 512. A step-up transformer 576 is also located on the shore-facing side of the subsea establishment structure 512. The step-up transformer 576 increases the voltage of the power produced in the turbine 544 for transmission to shore. A disconnector 578 is located proximate to the step-up transformer 576 on the second deck 534. The disconnecting unit 578 disconnects the power generation system of the undersea stand-by structure 512 from the transmission line 516 .

[0079] To begin power generation by OTEC plant 500, start-up generator 574 is turned on, powering seawater pumps 570-573, drawing seawater into subsea foundation structure 512 and initiating the heat exchange process between the seawater and ammonia. Once the ammonia gas begins to spin turbine generator 544 at a level that produces enough electricity to power subsea foundation structure 512, start-up generator 574 may be turned off. In response to receiving a demand signal from an onshore operations center, start-up generator 574 can be rapidly restarted to provide operating reserve and rapid load pickup to the utility grid.

[0080] In operation, the subsea establishment structure 512 generates power from the flow of cold and warm seawater. Warm seawater is pumped into the subsea establishment structure 512 from an area near the ocean surface adjacent to the subsea establishment structure 512 via warm water intakes 580, 581. The warm seawater, at a temperature of approximately 78-86°F, is drawn from a depth of approximately 24-40 feet below the ocean surface. The warm water is strained in strainers 568-569 and pumped through evaporative heat exchangers 548-549. In the evaporative heat exchangers 548-549, heat is transferred from the warm seawater to the liquid ammonia present in the cartridges of the evaporative heat exchangers 548-549. As the ammonia accepts the heat, it changes phase from liquid to gas. The gaseous ammonia is pumped to four turbine generators 544, rotating them and producing electrical energy. Electrical energy from turbine generator 544 is used to power subsea establishment structure 512 (e.g., on-board pump motors, electrical equipment, communication and control systems, lights, and devices). The remainder of the electrical energy produced within subsea establishment structure 512 is transmitted via transmission line 516 to onshore interconnection facility 510.

[0081] After the ammonia gas exits the turbine generator 544, it flows into cartridges in condensing heat exchangers 546-547. Cold seawater at a temperature of approximately 40°F is pumped from deep ocean water, filtered in strainers 566-567 through primary seawater pipe 522, and sent into condensing heat exchangers 546-547. The cold seawater cools the gaseous ammonia, causing it to transition from a gas back to a liquid. Liquid ammonia is collected in a tank directly below the condensing heat exchangers 546-547 and pumped back into evaporating heat exchangers 548-549, continuing the process in a closed loop. Thus, ammonia is never intentionally released into the air or water as a working fluid.

[0082] Subsea establishment structure 512 employs multiple pumps 570-573 so that maintenance can be performed on one of pumps 570-573 with minimal reduction in net power output. Seawater pumps 570-573 operate continuously at combined rates of 200,000 gpm to 500,000 gpm for warm surface ocean water and 170,000 gpm to 410,000 gpm for cold deep ocean water. Turbogenerator 544 is connected so that either heat exchangers 546-549 or turbinegenerator 544 can be isolated and removed offline for maintenance without interrupting the remaining plant operation.

[0083] The cycle of evaporating and condensing ammonia to produce electrical energy is monitored from a control room 552 in a crew area 542 on the first deck 532. The crew area 542 on the first deck 532 can be accessed from the upper main area 540 on the first deck 532 via a staircase. Many of the mechanical and electrical components of the power generation system in the subsea establishment structure 512 include sensors, video monitors, controls, and alarms that feed into a central control panel in the control room 552. Communications are available between the control room 552 and critical machinery spaces on the first deck 532 and the second deck 534. Communications are also available between the subsea establishment structure 512 and the interconnection facility 510 on land.

[0084] Emergency systems for dealing with fires, leaks, etc. are included in the control protocols for the subsea establishment structure 512. In the unlikely event of an ammonia leak within any space within the subsea establishment structure 512, sensors would detect the leak and sound an alarm, and if the risk exceeds a defined level, a medium-pressure water spray system would be activated. Ammonia has a very high affinity for water, and the aqueous ammonia solution produced from the water spray mixing with the ammonia would be collected in a separate gravity waste collection system. The water would be checked for environmental compatibility, treated as necessary, and then released. Figure 12 shows the undersea establishment structure 512 of Figure 5 as it would appear from shore. The undersea establishment structure 512 may be painted to match the sea and / or sky to limit the structure's visual impact from shore. The octagonal shape of the undersea establishment structure 512, along with the rectangular shaped shoreline, smooths the visual profile of the undersea establishment structure 512.

[0085] All references mentioned herein are incorporated by reference in their entirety.

[0086] Other embodiments are also within the scope of the following claims. For example, some OTEC plants also include a water supply line extending from the subsea foundation structure 112 to land. Such a water supply line can be used to provide cold seawater to an onshore facility for cooling. The cold water can be diverted before or after it passes through a condenser in the subsea foundation structure 112.

[0087] Some heat exchanger cabinets are arranged two racks per tier (four arrays high). In some heat exchangers, the side length may be reduced because the chamber is less deep and occupies less space. The reduced side length may also reduce loads caused by waves from passing (mega-panamax) cargo ships and from tsunamis. Pumps may also be arranged further (deeper) below the waterline in the dry machinery space.

[0088] Some OTEC plants use 3,000 mm diameter high density polyethylene (HDPE) pipes, which reduce pumping parasitic loads and / or flow expansion so that they can operate with only one set of pipes rather than two sets.

[0089] Some OTEC plants use micropiles rather than the standard 36- to 60-inch diameter piles. Micropiles can be installed or used by local contractors, thereby increasing the speed of installation and reducing the cost of installation.

Claims

[Claim 1] The invention described in this specification.

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